An organic field effect transistor (OFET) is such that an organic semiconductor is used in an active layer, and is expected to be applied to flexible electronics such as an active matrix display and an electronic paper. OFET is a basic constituent unit of these flexible electronics, and has a principle of controlling the current flowing between a drain electrode and a source electrode by the voltage applied to the gate electrode. When the gate voltage is increased, the amount of the current flowing between the drain and source electrodes is also increased. For high-speed operation of a transistor, high electric charge carrier mobility of a semiconductor and high on/off current ratio are needed. Coating-type OFETs, which can be produced by a solution process such as spin coating or printing, have attracted attention since they can realize more flexibility, a lower cost and a larger area which have conventionally been unattainable, and organic semiconductors which affect the performance of OFETs have been actively developed.
Device characteristics and operation stability of OFETs are known to be affected also by a gate insulating film because electric charge carriers are produced and moved in the vicinity of an interface between the organic semiconductor and the gate insulating film, and the performance aspect of the gate insulating film is also regarded as important. Especially, the basic characteristics of OFETs, such as mobility, threshold voltage and on/off ratio, are known to strongly depend on the smoothness and surface state of the gate insulating film. The respective properties such as the following are required of the gate insulating film: (1) hydrophobicity of the film surface; (2) high smoothness of the film surface; and (3) high insulation properties.
Insulating films including polymethylmethacrylate (PMMA), polyvinylphenol (PVP), polyimide or the like are publicly known as a gate insulating film. In addition, an insulating film including PVP and poly(melamine-co-formaldehyde) is known (see Patent Document 1).